Min-Ook Kim

Aligned Carbon Nanotube Arrays for Degradation-Resistant, Intimate Contact in Micromechanical Devices

Figure 1 . a) Schematic of the three-terminal micromechanical switch with aligned CNT arrays as the contact material. b) Onand off-states of the contact according to the actuation of the shuttle. c) Optical microscopy images of the contact region of CNT arrays in the onand offstates according to the applied gate voltage. The CNT arrays are in contact under V G = 0 V, and the current fl ows through the contact (the on-state). At V G = 52 V, the shuttle is driven in the direction of the gate electrode and the CNT arrays are separated (the off-state). The inset shows the corresponding relationship between I SD and V G . The scale bar is 10 μ m. The emergence of and advances in nano/ microelectromechanical systems (N/ MEMS) [ 1 , 2 ] have enabled the realization of miniaturized mechanical switches with high functionality in terms of low power consumption, fast operating speed, and low cost of fabrication. Various solid contact materials have been investigated for nanoand micromechanical contacts, including dopedSi, [ 3 , 4 ] single-crystal SiC and diamond, [ 5 , 6 ] and diverse metals and their alloys. [ 7 ] However, regardless of the contact material, these solidto-solid contacts suffer from diverse failure mechanisms such as stiction by adhesion, electrical short by welding and melting, unavoidable surface degradation by mechanical wear and abrasion, and a small real area of contact due to surface asperity. Therefore, the low reliability and durability of contacts are major challenges to overcome in nanoand micromechanical switching devices. Espe-

A flexible hybrid strain energy harvester using piezoelectric and electrostatic conversion

A new design of flexible energy harvester to utilize piezoelectric and electrostatic energy conversion mechanisms simultaneously from a single mechanical energy source is proposed. This non-resonant type harvester enables low-frequency mechanical inputs to be converted to electricity, and the polymeric structures make the harvester mechanically flexible, allowing it to be applied to non-planar surfaces. The fabricated harvester generated peak- and average power densities of 159 and 1.79 μW cm−2 respectively by piezoelectric conversion, and 52.9 μW cm−2 and 1.59 nW cm−2 respectively by electrostatic conversion from an input force of 1.2 N at 3 Hz. Considering its flexibility and ability to harvest mechanical inputs at frequencies below 3 Hz, low-frequency human movements could be a potential energy source for the proposed hybrid harvester to exploit.

Microswitch with self-assembled carbon nanotube arrays for high current density and reliable contact

A microswitch based on self-assembled carbon nanotube (CNT) arrays as micromechanical contact material has been demonstrated. The aligned CNT arrays are synthesized on microelectrodes and movable shuttle fabricated on a silicon-on-insulator (SOI) wafer. The CNT arrays are self-assembled on microstructures making mechanical contact between source and shuttle, and this contact is preloaded by the growth force of CNT and mechanical restoring force of the strained springs. Based on high interfacial contact area of CNT arrays, high electrical conductivity and excellent mechanical properties of CNT, highly reliable and durable micromechanical contact for microswitch is experimentally demonstrated while high-density currents are stably transmitted.

Development of flexible tactile sensor based on contact resistance of integrated carbon nanotubes

We have developed a novel three dimensional tactile sensor based on vertically aligned carbon nanotubes. The carbon nanotubes were directly synthesized on silicon microstructures and these CNTs-on-microstructures were integrated to flexible polydimethylsiloxane layers. Each tactile sensor has four sensing parts and the direction of force can be detected by monitoring the increase or decrease of electrical resistance in each sensing part. High gauge factor up to 272 and fast response less than 10 ms have been experimentally verified from the presented tactile sensor. The deviated contact resistance change from the initial value was less than 3% after repeated force input of 15 mN for 180,000 cycles.

Widely Tunable Variable Capacitor With Switching and Latching Mechanisms

A widely tunable variable capacitor using mechanical switching and a reversible latching mechanism was developed. This variable capacitor can increase the total capacitance by utilizing three discrete switches to sequentially connect four sets of fixed capacitors arranged in parallel. Continuous fine tuning is achieved by closing gaps with these interdigitated capacitors, and connected states are maintained through the use of a mechanical latching mechanism. By combining switching and gap-closing modes, a maximum tuning ratio of 9.42 was obtained.

Multidirectional flexible force sensors based on confined, self-adjusting carbon nanotube arrays

We demonstrate a highly sensitive force sensor based on self-adjusting carbon nanotube (CNT) arrays. Aligned CNT arrays are directly synthesized on silicon microstructures by a space-confined growth technique which enables a facile self-adjusting contact. To afford flexibility and softness, the patterned microstructures with the integrated CNTs are embedded in polydimethylsiloxane structures. The sensing mechanism is based on variations in the contact resistance between the facing CNT arrays under the applied force. By finite element analysis, proper dimensions and positions for each component are determined. Further, high sensitivities up to 15.05%/mN of the proposed sensors were confirmed experimentally. Multidirectional sensing capability could also be achieved by designing multiple sets of sensing elements in a single sensor. The sensors show long-term operational stability, owing to the unique properties of the constituent CNTs, such as outstanding mechanical durability and elasticity.

Reversible and continuous latching using a carbon internanotube interface.

Mechanical multistability is greatly beneficial in microelectromechanical systems because it offers multiple stable positioning of movable microstructures without a continuous energy supply. Although mechanical latching components based on multistability have been widely used in microsystems, their latching positions are inherently discrete and the number of stable positions is quite limited because of the lithographical minimum feature size limit of microstructures. We report a novel use of aligned carbon nanotube (CNT) arrays as latching elements in a movable micromechanical device. This CNT-array-based latching mechanism allows stable latching at multiple latching positions, together with reversible and bidirectional latching capabilities. The latching element with integrated CNTs on the sidewalls of microstructures can be adopted for diverse microelectromechanical systems that need precise positioning of movable structures without the necessity of continuous power consumption.

Variable capacitor with switching mechanism for wide tuning range

We developed a variable capacitor with mechanical switching mechanism and reversible mechanical latching component to enhance tuning ratio. The switching mechanism could connect four sets of capacitors arranged in parallel sequentially by controlling the displacement of a microactuator for abrupt and coarse tuning of total capacitance. Continuous and fine tuning was also achieved by gap-closing mode of interdigitated capacitors. The resultant maximum tuning ratio was 5.71 by combining coarse and fine tuning.

Continuously latchable shuttle using carbon nanotubes on sidewall surfaces

We demonstrated a novel usage of self-adjusted, vertically aligned carbon nanotube (CNT) arrays integrated on the sidewalls of microstructures as latching components. The CNT array-based latching mechanism showed stable latching at multiple latching positions, together with reversible and bidirectional latching capabilities. The latchable shuttle using CNT latch could be adopted for diverse microelectromechanical systems (MEMS) that need precise positioning of movable structures without the necessity of continuous power consumptions to hold the displaced position.